A common technique to increase the transmission capacity of today optical communication systems is wavelength division multiplexing (WDM), wherein a plurality of optical channels, each having a respective optical frequency (and correspondingly respective optical wavelength), are multiplexed together in a single optical medium, such as for example an optical fiber. The optical frequencies allocated for the WDM channels are typically arranged in a grid having an equal spacing between two adjacent frequencies. In dense WDM (DWDM), wherein the WDM channels may be closely spaced, the frequency spacing is typically equal to about 100 GHz (corresponding to a wavelength spacing of about 0.8 nm in the near infrared band—roughly between 1 μm to 2 μm) or about 50 GHz (about 0.4 nm in wavelength). Other used WDM channel separations are 200 GHz, 33.3 GHz and 25 GHz. Typically, the set of allocated optical frequencies occupies an optical bandwidth of about 4 THz, which gives room for the use of up to 40 or 41 WDM channels having 100 GHz spacing. The device of the present invention is suitable for a WDM optical bandwidth of at least about 1 THz, preferably at least about 2 THz, typically placed around 1550 nm.
Optical networking is expected to be widely used in perspective optical communication field. The term ‘optical network’ is commonly referred to an optical system including a plurality of point-to-point or point-to-multipoint (e.g., metro-ring) optical systems optically interconnected through nodes. In all-optical transparent networks few or no conversions of the optical signal into electrical signal, and then again in optical signal, occur along the whole path from a departure location to a destination location. This is accomplished by placing at the nodes of the optical networks electro-optical or optical devices which are apt to process the optical signal in the optical domain, with limited or no need for electrical conversion. Examples of such devices are optical add and/or drop multiplexers (OADM), branching units, optical routers, optical switches, optical regenerators (re-shapers and/or re-timers) and the like. Accordingly, the term ‘optical filtering’ or ‘optical processing’, for the purpose of the present description is used to indicate any optical transformation given to an optical radiation, such as extracting a channel or a power portion of said channel from a set of WDM channels (‘dropping’), inserting a channel or a power portion of said channel into a WDM signal (‘adding’), routing or switching a channel or its power portion on a dynamically selectable optical route, optical signal reshaping, retiming or a combination thereof. In addition, optical systems, and at a greater extent optical networks, make use of optical amplifiers in order to compensate the power losses due to fiber attenuation or to insertion losses of the optical devices along the path, avoiding the use of any conversion of the optical signal into the electrical domain even for long traveling distances and/or many optical devices along the path. In case of DWDM wavelengths, all channels are typically optically amplified together, e.g. within a bandwidth of about 32 nm around 1550 nm.
In optical systems, and at a greater extent in optical networks, a problem exists of filtering one or more optical channels at the nodes while minimizing the loss and/or the distortion of the filtered optical channel(s), as well the loss and/or the distortion of the optical channels transmitted through the node ideally without being processed (hereinafter referred to as ‘thru’ channels). Advantageously, the optical processing node should be able to simultaneously process more than one channel, each one arbitrarily selectable independently from the other processed channels. Ideally up to all the channels may be simultaneously selectable to be processed, but in practice a number between 2 and 16, preferably between 4 and 8, is considered to be sufficient for the purpose.
It is desirable that the optical processing node is tunable or reconfigurable, i.e., it can change dynamically the subset of channels on which it operates. In order to be suitable to arbitrarily select the channel to be processed within the whole WDM optical bandwidth, the tuning range of the whole optical processing node should be at least equal to said optical bandwidth.
It is also preferred that while the processing node “moves” from an initial channel (A) to a destination channel (B), the channels different from A and B remain unaffected by the tuning operation. In this case the component is defined as ‘hitless’. In particular, the channels placed between the initially processed channel and the final channel after tuning should not be subject to an additional impairment penalty, called ‘hit’, by the tuning operation. The hit may include a loss penalty and/or an optical distortion such as phase distortion and/or chromatic dispersion.
For example, optical communication networks need provisions for partially altering the traffic at each node by adding and/or dropping one or several independent channels out of the total number. Typically, an OADM node removes from a WDM signal a subset of the transmitted channels (each corresponding to one frequency/wavelength), and adds the same subset with a new information content, said subset being dynamically selectable.
There are several additional concerns. The tunable optical processing node should not act as a narrow band filter for the unprocessed channels, since concatenation of such nodes would excessively narrow the channel pass bands. The tunable optical processing node should also be ultra-compact and should have low transmission loss and low cost, since these important factors ultimately determine which technology is selected.
In article “Non-blocking wavelength channel switch using TO effect of double series coupled microring resonator”, S. Yamagata et al., El. Lett. 12 May 2005, Vol. 41, No. 10, it is demonstrated a non-blocking tunable filter using the thermo-optic (TO) effect of a double series coupled polymer microring resonator by controlling individual resonant wavelengths.
In article “Fast and stable wavelength-selective switch using double-series coupled dielectric microring resonator”, Y. Goebuchi et al., IEEE Phot. Tech. Lett., Vol. 18, No. 3, Feb. 1, 2006, it is demonstrated a hitless tunable add-drop filter using the thermo-optic effect of double series coupled dielectric microring resonator.